Short answer is "within half-scale deflection" of the CDI.
Generally, to be established on a segment implies all of the below:
1. You've passed the segment's defining fix
2. You are receiving "positive course guidance" (PCG) from the navigation system
3. You are within half-scale deflection on the CDI **
4. You are "stable" in the segment (i.e., with a positive trend to track the centerline)
** The "+/- 5 degrees" is mostly for NDBs (since there's no CDI).